The present invention relates to a tablet input apparatus having a tablet in which a segment including at least one lattice point is formed on a matrix comprising a group of conductors arranged in spaced relation along coordinate axes, wherein an input pen is placed in contact with a desired segment of said tablet to put out said segment as coordinate data, and more specifically, to a tablet input apparatus which is free of erroneous operation or incapability of detection in the event that an input pen is placed in contact with a segment having more than two lattice points.
Tablets internally provided with lattices of parallel conductors in regularly spaced relation along coordinate axes are described, for example, in U.S. Pat. No. 3,567,859 and U.S. Pat. No. 3,732,369. In these tablets, one set of a group of conductors are sequentially electrically pulsated and the other set of a group of conductors are then pulsated. An input pen having a tip formed as a conductor is placed in contact with said tablet to provide capacitive coupling at two sets of conductor lattice points. Thereby, a signal at the input pen from the pulsated conductor is used to digitalize the coordinate of a position of the pen in the surface of the tablet. By this operation, the position of the pen along one coordinate is digitalized, after which the other coordinate position is digitalized. The tablets of these U.S. Patents are designed for the purpose of inputting a handwritten figure into a computer, and therefore, the spacing between two sets of a group of conductors is very narrow thereby improving the resolution of the locus of the input pen.
However, recently, there is proposed an arrangement wherein an information sheet of a tablet is divided into segments, and reference symbols peculiar to each segment are assigned thereto, which tablet is used as a key board, as disclosed, for example, in U.S. Pat. Nos. 3,974,332 or 4,136,336. In these tablets or key boards, the segment indicates one "key" region, which substantially functions to form a digital signal representative of one coordinate position. Accordingly, in said arrangement, the locus of a pen is not converted into a series of digital signals but a position of the pen which moves at random from one segment to the other may be digitalized separately and independently. Therefore, unlike the tablet for inputting of a figure as described above, the spacing between two sets of a group of conductors can be made wide, and also the width of each conductor can be made wide. In these tablets, there is no limitation peculiar to the segments thereof, that is, the number of keys, and therefore, the tablets are suitably used for the purpose of inputting information having many kinds, for example, Japanese or Chinese, into a computer. Furthermore, in case a stock of several thousands of parts or chemicals is controlled by a computer, the aforesaid tablet is extremely advantageously used to input codes of such parts.
The above-described tablet input device comprises a first scanning unit for sequentially applying a scanning pulse to a group of conductors in an X axis of coordinate axes and a second scanning unit for sequentially applying a scanning pulse to a group of conductors in a Y axis, wherein an input pen provides an output signal every time a scanning pulse is applied to a conductor in a segment with which the input pen is placed in contact. The tablet input device comprises a counter for counting scanning pulses and the value counted by the counter indicates the coordinate of a key segment with which the input pen is placed in contact. The scanning pulses are supplied two by two periods from each of scanning units in order to obtain accurate discrimination and to remove influences caused by external noises or the like to enhance reliability of data input. Thus, a first detection signal obtained by the first scanning pulse is applied as a latch strobe signal to a latch circuit, and the value counted by the counter at that time is stored. On the other hand, a second detection signal obtained by the second scanning pulse is applied as a comparing strobe signal to a comparison circuit. The comparison circuit compares the value counted by the counter with a data latched by the latch circuit at timing the comparing strobe signal is input, and when they coincide with each other, the data of the latch circuit is output as a coordinate data to the information processing apparatus.
In such tablet input apparatus, it is sometimes required that an area of some segment is increased to display functions, phrases, etc. thereon in order that information displayed on the information sheet on the tablet may be seen easily. In this case, one enlarged segment includes more than two lattice points and when the input pen is placed in contact with a central portion of the enlarged segment, the tip of the input pen becomes positioned in the central portion between the lattices. Therefore, the input pen provides an output signal every time scanning pulses are applied to two conductors adjacent to the input pen and a first and a second detection signals are obtained by the application of the first scanning pulse. However, the first detection signal is different from the second detection signal in the value counted by the counter, no coincidence is attained by the comparison circuit and the coordinate data is impossible to be put out, resulting in formation of a so-called non-detection zone.